A method and network system for implementing user port locating in multi-machine backup scenario of broadband remote access server (BRAS) are disclosed. The method comprises: configuring master port in each backup group as user access port and synchronizing information of taking the maser port as user access port to the backup port in the backup group; a remote authentication dial in user service (RADIUS) server configuring access port information, which is set as the master port in the backup group, for the user; processing master-backup switch if the master port is abnormal and the user accessing the line from the corresponding backup port; the BRAS corresponding to the backup port encapsulating the authentication message including the access line information about digital subscriber line access multiplexer of the user access and taking the maser port as the user access port, and sending the authentication message to the RADIUS server.